The present invention relates generally to examining insulated electrical conductors of the type having either a semiconductive layer provided on a central core of stranded wire, or a conductor having a second layer of semiconductor material disposed on the outer surface of a relatively thick jacket of insulating material located on the first semiconductive layer.
In U.S. Pat. No. 4,260,566 to Brouwer et al, an apparatus is shown for detecting imperfections in a semiconductive shield applied to an electrical conductor. An AC current is applied between a bare portion of the conductor and apparatus applying a jacket of insulating material over the strand shield as the conductor and shield move through the apparatus. The impedance of the conductor and apparatus is measured to provide an indication of changes in the quality of the strand shield applied to the conductor.
U.S. Pat. No. 2,548,598 to Feiker discloses the use of a cavity with two, opposed chokes for measuring imperfections on the surface of a bare metal wire directed through the chokes and cavity. Resistive flaws impede surface currents along the wire and inside the chokes, which lower the Q of the resonant cavity, i.e., the cavity appears more lossy. The wire in the chokes forms a coaxial transmission line, the characteristic impedance of which is changed by nicks on the wire surface, which detunes the cavity.
U.S. Pat. No. 3,739,263 to Henoch discloses the use of a cavity structure, designed to resonate at two distinct frequencies, to measure the thickness of a dielectric layer on a metal wire. A swept frequency oscillator feeds the cavity while a first detector measures the diameter of the wire before the layer is applied and a second detector measures the diameter of the wire after the layer is applied. The two diameters affect the two resonant frequencies in the respective cavities in a manner that provides an indication of the thickness of the layer.